Abstract: An optical semiconductor device includes: a base including a surface intersecting with a first direction; a mesa protruding from the surface in the first direction, including a top surface and two side surfaces, and extending along the surface in a direction intersecting the first direction; and a heater layer including a top wall positioned on a side opposite to the base with respect to the top surface, the heater layer extending along the mesa, the mesa including a first mesa extending in a second direction intersecting the first direction, and a plurality of second mesas branching from the first mesa and extending so as to be away from each other in a third direction toward the second direction from the first mesa, the third direction intersecting both of the first direction and the second direction.